1. Field of the Invention
This invention relates to waveguides that collect and condense light from a light source, transforming the area and divergence angle of the light from their input to their output with minimum loss of brightness.
2. Description of the Related Art
The objective of systems that collect, condense, and couple electromagnetic radiation into a target such as a standard waveguide, e.g. a single fiber or fiber bundle, or output electromagnetic radiation to the input of a projection engine, is to maximize the brightness of the electromagnetic radiation at the target. There are several common systems for collecting and condensing light from a lamp for such illumination and projection applications.
One optical collection and condensing systems, U.S. patent application Ser. No. 09/604,921 now U.S. Pat. No. 6,672,740, the disclosure of which is incorporated by reference, provides a dual-paraboloid reflector system. This optical collection and condensing system, as illustrated in FIG. 1(a), uses two generally symmetric paraboloid reflectors 10, 11 that are positioned so that light reflected from the first reflector 10 is received in a corresponding section of the second reflector 11. In particular, light emitted from a light source 12, such as an arc lamp, is collected by the first parabolic reflector 10 and collimated along the optical axis toward the second reflector 11. The second reflector 11 receives the collimated beam of light and focuses this light at the target 13 positioned at the focal point.
The optical system of FIG. 1(a) may employ a retro-reflector 14 in conjunction with the first paraboloid reflector 10 to capture radiation emitted by the light source 12 in a direction away from the first paraboloid reflector 10 and reflect the captured radiation back through the light source 12. In particular, the retro-reflector 14 has a generally spherical shape with a focus located substantially near the light source 12 (i.e., at the focal point of the first paraboloid reflector) toward the first paraboloid reflector to thereby increase the intensity of the collimated rays reflected therefrom.
In FIG. 1(a) is shown light paths for three different rays (a, b, and c) emitted from the light source 12 when viewed in a direction normal to the lamp axis. The light output from a lamp subtends an angle of about 90° around an axis normal to the lamp, as indicated by rays a and c in FIG. 1(a).
The light output from a lamp subtends a cone angle of nearly 180°, on the other hand, when viewed in a direction parallel to the lamp axis, as indicated by rays a′ and c′ in FIG. 1(b).
One shortcoming of the above described on-axis, dual-paraboloid optical system is that a large angle is produced between rays a and c, and rays a′ and c′, at a target. As a result, the rays strike the target 13 at a high angle of incidence relative to the target surface. Thus, the numerical aperture (NA) at the input of the target 13 may be very large, sometimes as high as 1.0, while the area upon which the light is focused is small. A large numerical aperture combined with a small area may be unsuitable for the optical components to which light from the system may be coupled. If a different, e.g. smaller, numerical aperture is desired, some means of transforming the area and divergence angle of the light with minimum loss of brightness may be incorporated into the device.
Representative means of transforming input areas and divergence angles of light are lenses and tapered optical waveguides, also known as tapered light pipes (TLP). While lenses provide an efficient means of transforming input areas and divergence angles of light, they require a certain amount of space in which to operate. Also, they are not well adapted to large numerical apertures. Consequently, tapered light pipes are often used instead of lenses. Tapered light pipes, however, must be relatively lengthy to transform light efficiently.
In U.S. application Ser. No. 09/669,841 now U.S. Pat. No. 6,634,759, the disclosure of which is incorporated by reference, a dual ellipsoidal reflector system is described as providing 1:1 magnification for small light source target. This optical collection and condensing system, as illustrated in FIG. 2, uses two generally symmetric ellipsoid reflectors 20, 21 that are positioned so that light reflected from the first reflector 20 is received in a corresponding section of the second reflector 21. In particular, light emitted from the light source 22 is collected by the first elliptical reflector 20 and focused onto the optical axis 25 toward the second reflector 21. The second reflector 21 receives the focused beam of light and refocuses this light at the target 23 positioned at the focal point.
As may be seen in FIG. 2, the dual-ellipsoid system suffers from the same disadvantage as the dual-paraboloid system in that a large angle is produced between ray a and ray c at the target. As a result, ray a and ray c also strike the target at large angles of incidence relative to the target surface, requiring further transformation of the input area and divergence angle of the light.
Another embodiment of the dual-ellipsoid system may be seen in FIG. 3. This dual-ellipsoid system suffers from the same disadvantage as the above-mentioned dual-paraboloid and dual-ellipsoid systems in that a large angle is produced between ray a and ray c at the target. Here too, ray a and ray c strike the target at large angles of incidence, requiring further transformation of the input area and divergence angle of the light.
In practice, light with such a large NA may be transformed such that the NA is smaller and the area is larger following the brightness principle. The transformation may be performed with, e.g. a tapered light pipe.
A standard long tapered light pipe 40a with a flat input surface 41a for use with the above systems is shown in FIG. 4(a). A standard short tapered light pipe 40b with a flat input surface 41b for use with the above systems is shown in FIG. 4(b). Both the long and the short tapered light pipes may be used to transform light having a small area d1 and large numerical aperture NA1 at the input 41 to a larger area d2 and smaller numerical aperture NA2 at the output 42. If light 43 impinges the tapered light pipe 40 at large angles of incidence 44 as shown in FIG. 4, the tapering of the light pipe 40 will transform the large input angles 44 into smaller output angles 45. The degree to which the angles are transformed will depend on the degree of taper. For ideal tapered light pipes, brightness is conserved. Consequently, for an ideal tapered light pipe, the product of the numerical aperture NA1 and the area d1 of the light at the input 41 will be equal to the product of the numerical aperture NA2 and the area d2 of the light at the output 42. To wit:
 d1*NA1=d2*NA2  (1)
In actual implementation, optimizations need to be performed such that the optimized dimensions may deviate from the ideal configurations.
The output angles 45 are designed for a specific system by matching the tapered light pipe to an output device. In designing a tapered light pipe, three of the variables will often be known, and the fourth can be calculated. In one example, a tapered light pipe of length 75.0 mm was designed with d1=3.02 mm, NA1=0.7, and d2=9.0 mm. The output numerical aperture NA2 is thus predicted to be 0.23. Upon fabricating the tapered light pipe, however, the actual numerical aperture at the output was found to be 0.26, larger than the predicted 0.23. Such a large numerical aperture will result in a loss of coupling efficiency in subsequent optical elements. But if the input area is reduced to reduce the numerical aperture at the output, less light will be coupled into the tapered light pipe in the first place, reducing the overall collection efficiency of the system.
The reason the numerical aperture at the output is larger than predicted is due to an assumption underlying equation (1) to the effect that an ideal tapered light pipe is of infinite length. For a tapered light pipe of infinite length, the angle of taper would be zero. In actuality, however, the angle of taper must be some number larger than zero, since the tapered light pipe is of finite length, and so the actual numerical aperture differs from that predicted by the equation. As the tapered light pipe gets longer, the actual numerical aperture converges to the predicted numerical aperture. A longer tapered light pipe, however, may require more space.
Furthermore, when the output numerical aperture of a tapered light pipe such as those shown in FIG. 4 was measured by placing a pinhole against the output face, an angle shift was observed which indicated that the output light may not be telecentric.
In FIG. 5 is shown a radiation envelope of a typical arc lamp. Radiation tends to be emitted by an arc lamp in a pattern that subtends an angle of ±90° in a plane parallel to the axis of the lamp (z-axis in FIG. 5), and 360° around the axis of the lamp. If the envelope were projected along the z-axis onto a flat surface, it would appear to be circular. Light focused at the target of a dual paraboloid or dual ellipsoid reflector configuration with retro-reflectors from such a lamp may, e.g. have an elliptical numerical aperture (NA) that varies from 1.0 in the z-direction to 0.7 in, e.g. the x-direction.
A numerical aperture (NA) of a system, such as the dual paraboloid system shown in FIG. 1(a) may, however, be rectangular as shown in FIG. 6, rather than circular or elliptical. The NA along the diagonal of a cross-section of an input surface may thus be larger than the NA in either the x or z directions. When the light is transformed by, e.g. a TLP, a similar rectangular or square angular distribution may be obtained at the output, as shown in FIG. 7, which is a square in this example. Since the radiation input to the system has a circular or elliptical distribution, however, a circular NA, such as the one shown in FIG. 8, or an elliptical NA may be more appropriate for common optical systems.
FIG. 9 shows various configurations of input apertures for a target. The input apertures generally have aspect ratios greater than one. The aspect ratios of the input apertures may thus be made to be similar to the aspect ratio of the emission area of an arc lamp viewed from the side. Matching an input aperture at the target to an arc, however, does not necessarily match it with the final output device, e.g. a fiber or projection engine. It would be desirable, therefore, for a transforming device to transform the aspect ratio and the NA of the input light into a satisfactory aspect ratio and NA for the output device.
Therefore, there remains a need to provide an efficient means of transforming input area and divergence angle of light, in a relatively short space, such that the output is telecentric and has a circular or elliptical NA distribution that may be symmetric.